This invention relates to optical communications networks. The invention is particularly directed to point-to-point connections in which a protection path is provided, to enable communication between a pairs of communicating nodes despite rupture of the main signal carrying fibre between those nodes.
One possible way to provide a protection path between nodes is to provide an additional protection cable between the pair of nodes, which preferably follows a different path to the main signal carrying fibre joining the nodes. This ensures that the protection cable is not susceptible to the same failure as the main cable. The source node provides the data to be protected on two ports- a working port and a protection port, and the signals from the two ports are routed using the different paths. These paths may pass through other nodes wit a network, although the connection between the source and destination nodes may be considered as a point-to-point connection.
This type of dedicated protection scheme results in a large amount of additional bandwidth which is normally unused. This approach may therefore be considered to be bandwidth inefficient, although it is extremely simple to implement.
An alternative approach is to provide a protection path of lower bandwidth than the bandwidth of the main traffic between the nodes. This may be achieved by having a first number of main signal carrying fibers between the nodes, and a second lower number of protection fibers. For example, 1; n protection may be implemented, where n is the number of working fibers sharing a protection fiber. This system is slightly more complicated to implement, as the data on the working fiber or fibers must be prioritised so that the protection bandwidth can be allocated to the protection of the most important data in the event of a failure.
The invention is concerned with either type of protection scheme in point-to-point communication links.
In normal operation of the communication link; all traffic will flow along the working fiber or fibers. At the receiving end of the link, the receiving circuitry is used to detect failure and thereby trigger the protection switching operation when necessary.
This protection switching involves operating a switch so that traffic from the protection fiber (instead of the working fiber) is routed to the receiver. It is, however, necessary also to monitor the protection path, as this may also be subject to failures. This can not be achieved with the receiving circuitry as in normal operation it only receives data from the working fiber or fibers.
In WDM systems, signals are modulated on a specific channel wavelength for transmission over the network. The channel wavelength is likely to be different to the frequency on which the signal from the client device is modulated. Therefore, it is known to provide wavelength translators (transponders) at the transmitting node and at the receiving node. These transponders carry out opto-electric and electro-optic conversion with frequency translation in the electrical domain. The transponders can also be used for detecting signal failure.
Therefore, in a known systems the signals from the transmitting node are divided into two branches and then pass through transponders on each branch. These enable different power levels to be injected into the main and protection paths, so that the power received at the receiving node is within required limits of the receiving circuitry. At the receiving node, there are transponders on each branch (the main branch or the protection branch) before the switch routes signals on the selected branch to the receiver. These transponders at the receiving node enable the protection path to be monitored for failures as well as the working path.
A problem with this configuration is the number of wavelength translators required at each node site.
According to the present invention, there is provided an optical network comprising a source node and a destination node, a first, main, optical fiber path between the nodes and a second protection, optical fiber path between the nodes, wherein each path is for carrying a plurality of WDM channels, wherein the some node is provided with a plurality of transponders and a splitter for providing source node data onto the main and protection paths, the transponders being positioned on the node side of the splitter, and where the destination node is provided with a plurality of transponders and a switch for selecting which of the main and protection paths to route to a receiver of the node, the transponders being positioned on the node side of the switch
This arrangement reduces the number of transponders required, as they are provided before the splitter at the transmitting node, and after the switch at the receiving node.
Preferably the destination node fiber comprises a monitoring arrangement for monitoring the signal from the protection path. The separate monitoring arrangement is provided for monitoring the receipt of data from the protection path, as the transponders at the receiving node cannot be used for monitoring the protection path.
The reduction in the number of transponders at the transmitting node removes the possibility of independent gain control for the main and protection paths using the transponders at the transmitting node. This may not cause any problem if the node amplifiers (pre-amplifiers and post-amplifiers) have sufficient gain flattening. However, the source node may comprise an optical attenuator for each channel of the main and protection paths. This enables the signal level at the receiving node on the main and protection paths to be controlled. These optical attenuators are additional components but they can be implemented at much lower cost than the transponders previously used for signal equalisation purposes. For example, the attenuators comprise variable optical attenuators.
The monitoring arrangement may comprise a wavelength switch for providing the signal on each wavelength of the protection path, in turn, to a receive. There is no need to monitor the channels simultaneously, as a slow monitoring arrangement is sufficient. A tuneable filter arrangement may instead be used to route protection channels selectively to a detector arrangement.
The switch may comprise a 2xc3x972 photonic switch, with the main and protection paths as inputs, and the node receiver and the monitoring arrangement as outputs. The transponders in the destination node are used for failure detection of data on the main path, with failure detection being signalled to a control unit which controls the operation of the switch.
The main fiber path may have greater bandwidth than the protection fiber path, so that protection is shared For example, the main fiber path comprises a first number of fibers and the protection fiber path comprises a lower second number of fibers. This enables 1:n or n:m protection to be implemented.
According to a second aspect of the invention, there is provided, in an optical communications network comprising a source node and a destination node, a first, main, optical fiber path between the nodes and a second, protection, optical fiber path between the nodes, with each path carrying a plurality of WDM channels, a method of sending protected data from the source node to the destination node, comprising
at the source node, passing the data from the source node to be allocated to a channel through a transponder for providing the data onto a WDM channel frequency;
splitting the data on the WDM channel frequency into main and protection components;
routing the main and protection components using diverse paths to the destination node;
at the destination node, using a switch to select which of the main and protection path signals are routed to the receiver of the destination node, monitoring signal failure or degradation on the channels of the incoming main path to the destination node using transponders in the receiver, and monitoring the signal received from the protection path using a monitoring arrangement.
This method enables a data channel to be split into main and protection pats after the wavelength translation operation. At the receiving node, wavelength translation is carried out after the switching operation, so that the transponders can only be used for monitoring signal failure on one of the main or protection paths. Thus, a separate monitoring arrangement is provided.
The main and protection components may be passed through variable attenuators, for equalising the signal intensity received along the and protection paths at the destination node.
The invention also provides a source node and a destination node for use in the network architecture of the invention.